1. Field of the Invention
The present invention relates to a tyre tread band, in particular for off-road motor-vehicles, of the type comprising at least two longitudinal grooves substantially parallel to each other and extending circumferentially on opposite sides of the equatorial plane of a tyre carrying said tread band; at least one series of transverse grooves crossing the longitudinal grooves, said transverse and longitudinal grooves delimiting at least two rows of shoulder blocks circumferentially distributed at opposite side edges, and at least one row of centre blocks disposed between said rows of shoulder blocks.
In more detail, the invention relates to a tread band having a pattern of a bidirectional and/or symmetric type.
The invention also pertains to a tyre, in particular for off-road motor-vehicles, provided with the tread band in reference.
2. Description of the Related Art
It is known that fread bands in tyres have a pattern suit-ably cut by a series of longitudinal and transverse grooves, adapted to normally define one or more rows of circumferentially distributed centre blocks alongside which, on respectively opposite sides, there are respective rows of shoulder blocks located close to the opposite side edges of the tread band.
The geometrical and size features of the grooves and of further possible cuts provided in the tread band, and consequently of the blocks defined thereby, are selected each time during the planning step, depending on the performance features required from the tyre, based on the intended type of use.
Usually, the longitudinal grooves perform the function of giving the tyre steering property and roadholding features in relation to transverse stresses, that is stresses directed parallelly to the rotation axis of the tyre. The transverse grooves in turn perform the function of giving the tyre good traction performance, in relation to the tangential thrusts directed parallelly to the running direction.
In addition, longitudinal and transverse grooves cooperate in carrying out an efficient water-draining action from the ground-contacting area of the tyre when the vehicle is running on a wet road surface.
After the above statements, in making tyres particularly adapted for use on off-road motor-vehicles, good traction performances are to be achieved above all with reference to muddy and snow-covered grounds or in any case grounds of low consistency. These requirements are presently met by providing a great number of grooves and longitudinal and transverse cuts of great width and depth.
However this solution has a tendency to impair the tyre running behaviour on normally asphalted roadways. Actually, due to the presence of many big grooves, the sizes and stiffness of the blocks is greatly reduced.
Consequently, the block can be unable to conveniently withstand stresses to which the tyre is submitted both when the vehicle is running on a straight stretch and during the cornering, thereby giving rise to a rather uncertain and imprecise behaviour on the road, as well as to early and/or uneven wear of the tread band. In addition, the presence of big transverse grooves has a tendency to give rise to an undesired increase in noise and in the rolling resistance force of the tyre and, as a matter of fact, the present market requirements attach an increasingly greater importance to these factors.
In accordance with the present invention, it has been found that an optimal running behaviour on an asphalted road can be given to a tyre while enabling the same to keep appropriate traction performances on muddy, snow-covered or in any case loose road surfaces, by arranging transverse grooves in the tread band consisting each of at least two channels of curved extension that, starting from respective side edges of the tread band mutually meet to form a continuous groove of undulated extension obliquely crossed by a cut made up of the inner end stretches of said channels and preferably slightly inclined to the circumferential extension direction, or more preferably substantially parallel thereto.
Still in accordance with the invention, the thread band of a tyre, in particular for off-road motor-vehicles, is characterized in that each of said transverse grooves is substantially defined by at least two transverse channels each of which has an outer end stretch extending from the respective side edge of the tread band according to an orientation substantially perpendicular to the equatorial plane; an inner end stretch preferably inclined to the equatorial plane; a curvilinear connecting stretch joining the outer end stretch to the inner end stretch; the connecting stretch of each transverse channel meeting the connecting stretch of the laterally-opposite transverse channel to give said transverse groove an axially continuous course also interconnecting the inner end stretches of each transverse channel in a continuity relationship.
Preferably, each transverse channel has a gradually decreasing width away from the corresponding side edge of the tread band and, in another aspect of the invention, it preferably terminates beyond the equatorial plane with a respective inner end stretch the axis of which is oriented at an angle included between 0xc2x0 and 25xc2x0, relative to said equatorial plane.
The outer end stretches of the transverse channels are in turn preferably inclined to an angle included between 0xc2x0 and 20xc2x0 on both sides relative to the direction normal to said equatorial plane.
Advantageously, the connecting stretches of the transverse channels having a curved extension with a radius of curvature included between 10% and 40% of the tread width, mutually meet close to the equatorial plane of the tyre and have their mutual-intersection points lying in a common straight line oriented according to an inclination included between 15xc2x0 and 45xc2x0 with respect to the equatorial plane.
In addition, it is preferably provided that auxiliary channels should be associated with each of said shoulder block rows, said auxiliary channels each being disposed at a position between and preferentially spaced apart substantially the same distance from the adjacent transverse channels.
Each auxiliary channel advantageously has a first stretch extending from the corresponding side edge of the tread band in a direction substantially normal to the equatorial plane and/or parallel to the outer end stretches of the adjacent transverse channels, and a second stretch crossing an adjacent transverse channel in a direction substantially parallel to the equatorial plane, intersecting said channel close to the transition point between said outer end stretch and connecting stretch.
More preferably, the axis of the second stretch in each auxiliary channel is inclined at an angle included between 0xc2x0 and 25xc2x0 relative to the equatorial plane and oriented in an opposite direction to the inner end stretch of the circumferentially adjacent transverse channels.
Each auxiliary channel further has an intermediate stretch connecting the first and second stretches by a curved course.
It is also preferably provided that at least one longitudinal cut should extend from the first stretch of each auxiliary channel, the longitudinal cut partly penetrating into an adjacent shoulder block.
Advantageously, the transverse and auxiliary channels define first shoulder blocks each of which comprises an outer portion of an elongated conformation in a direction substantially perpendicular to the equatorial plane and an inner portion of an elongated conformation in a direction substantially parallel to the equatorial plane, as well as second blocks of same configuration as the outer portions of the first blocks.
As regards centre blocks, each of them has a conformation substantially defined by a centre portion disposed diagonally at the equatorial plane and interconnected, at respectively opposite sides, with two side portions substantially parallel to the equatorial plane.
In accordance with another preferential feature of the present invention, each of said longitudinal grooves consists of a series of stretches, preferably substantially rectilinear, each extending between two contiguous transverse grooves at an angle included between 0xc2x0 and 10xc2x0 relative to said equatorial plane, all preferably inclined in the same direction.
In another aspect, the present invention relates to a tyre for motor-vehicle wheels, in particular off-road motor-vehicles, provided with a tread band having the above described features.
In a further aspect, the present invention relates to a tyre having a tread band, in particular for off-road motor-vehicles, comprising at least two circumferentially-extending longitudinal grooves; a plurality of transverse grooves parallel to each other, each comprising at least one first concave stretch and one second convex stretch relative to the circumferential direction of the tyre, to form a transversely continuous groove of undulated extension; said transverse and longitudinal grooves delimiting at least two rows of shoulder blocks circumferentially distributed at opposite side edges of the tread band and at least one row of centre blocks disposed between said rows of shoulder blocks, wherein at least one of said transverse grooves comprises at least one third stretch and one fourth stretch, said third and fourth stretches connected to at least one of said transverse grooves in a continuity relationship and extending in substantially opposite directions, substantially close to the connecting point between said first concave stretch and said second convex stretch.